1. Field of the Invention
The invention relates to the field of optical instrument focusing systems.
2. Prior Art
Prior art optical instrument focusing systems are of two kinds, mechanically aided, but operator controlled, and fully automatic.
Mechanically aided, operator controlled, focusing systems are of many varieties. These varieties include double image, split image and broken image systems. In each of these systems, the operator must adjust the focus of the optical system until the operator determines that the optimum image has been achieved. In double image systems, the elimination of the double image is the criterion which the operator must use. In split image systems, the operator must align separate portions of the image to achieve a unitary image. In broken image systems the operator must adjust the focus to yield an image which to the operator's eye is the clearest image.
Automatic focusing systems have utilized moving lenticular screens disposed between a photocell and a lens system at the location where the focus of the image is positioned when the optical instrument is in focus. Such systems make use of the fact that at the focus of an image projected by a lens system the image is a point. When the focus of the image is at the lenticular screen, the image passing through the lenticular screen suffers a minimum of variation in image intensity. However, when the focus of the image is displaced for the lenticular screen, the image passes through portions of the screen having differing focal points and the image is distorted. A photocell placed in the path of the image transmitted by the lenticular screen provides an output signal which is dependent on the position of the focus of the image relative to the lenticular screen. With a motor driven lens focusing system, the motor is automatically stopped at the lens position which is determined on the basis of the photocell output as corresponding to the best focus. Unfortunately, such systems have a disadvantage in that in order to establish that an image is in proper focus the lens must be moved out of focus because the system determines focus on a relative not an absolute basis. During focusing, the rate at which the lens focus is adjusted must be limited to a low enough rate that the optimum focus can be detected in time to stop the lens motion before the lens passes out of focus or else the focusing motor must be reversed to bring the image back into focus. Such systems have a further disadvantage that when the lens is adjusted by the motor, it cannot be determined a priority in which direction the motor should adjust the focus. Therefore, on the average one-half of the time the motor will begin by adjusting the lens focus in the wrong direction, in consequence, the motor must be stopped and reversed with attendant disadvantages with respect to vibration power consumption and focusing time.